The lines in which J does not change are weaker (“satellite” lines), and the lines in which the change in J is in the opposite sense to the change in L are the weakest (“second satellites”...The lines in which J does not change are weaker (“satellite” lines), and the lines in which the change in J is in the opposite sense to the change in L are the weakest (“second satellites”). It was mentioned in Chapter 7 that one of the tests for LS-coupling was Hund’s interval rule, which governs the spacings of the levels within a term, and hence the wavelength spacings of the lines within a multiplet.